The present invention relates to vehicle wheel speed sensing devices and more particularly, to sensing devices of the variable reluctance type having associating rotating exciter ring and fixed sensor components.
Variable reluctance sensors are used in a variety of speed and position sensing applications. Variable reluctance sensors are electro-magnetic devices that produce an output voltage due to a time varying change in flux flowing through the sensor's coil. This provides a means of measuring translational or rotational motion.
Wheel speed sensing devices such as those used in combination with vehicle anti-lock braking systems are known wherein a sensor provides a magnetic field through some type of magnet that is generally combined in proximity with a sensing coil into a single sensor component. The coil-magnet component, (sensor), is generally mounted in a fixed position relative to the vehicle since it includes signal wires for communication with an electronic control means.
Varying flux is caused by a rotating exciter ring that is positioned near the sensor. The exciter ring is generally made of iron and includes a toothed configuration. It is known to mount an exciter ring so as to turn in rotation at the angular speed of the wheel within the sensor's magnetic field to create an electric signal that varies with the speed of the wheel.
It is, in general, known to mount the sensing device such that it is combined with the wheel's bearing assembly, since the bearing assembly provides a fixed race and a rotating race, moving at wheel speed, in close proximity. In this type of a device the exciter ring is conventionally mounted to the rotating race and the sensor component is mounted to the fixed race. Combining a sensing device with the bearing provides an advantage in-that bearings typically have seals enclosing the bearing area including an annular space between the fixed and rotating races, protecting the bearing rolling elements and retaining lubricant. These advantages combine to make it preferential to mount the sensing device in combination with the wheel bearing assembly.
Preferably, the sensor element is installed such that it can be removed for servicing or replacement with minimal disassembly of other vehicle components and without a requirement for special tooling. This is complicated when the sensing device is mounted in association with a wheel bearing.
Additionally, it is essential that a proper air gap for the magnetic flux path route between the sensor and the exciter ring be maintained to provide an acceptable signal. Variations that occur in the wheel bearing, sensor and exciter ring as a result of manufacturing and assembly operations can result in undesirable air gap variations.
Therefore, due to the complication of assembly and tolerance stack of the wheel bearing, assembly obstacles must be overcome to mount a sensing device to take advantage of the protected environment between the wheel bearing seals. When mounting the sensing device in combination with the wheel bearing assembly, these and other obstacles that arise must be overcome to provide an acceptable sensing device and wheel bearing combination.